什么是继承上下文管理器的pythonic方式

Question

Python教我们使用__enter__和对对象进行清理__exit__.如果我需要创建一个使用对象的对象,必须使用上下文管理器怎么办？想象一下:

from database1 import DB1
from database2 import DB2

通常情况下,他们会这样使用:

with DB1() as db1, DB2() as db2:
    db1.do_stuff()
    db2.do_other_stuff()

无论发生什么,db1并db2都将运行自己的__exit__功能,并清理连接,同花顺等

当我把所有这些都放在课堂上时,我该怎么做？这是正确的吗？这显然是不正确的,正如评论中所指出的那样,上下文管理器db1并db2在块的末尾运行.

class MyApp(object):
    def __enter__(self):
        with DB1() as self.db1, DB2() as self.db2:
            return self
    def __exit__(self, type, value, traceback):
        self.db1.__exit__(self, type, value, traceback)
        self.db2.__exit__(self, type, value, traceback)

我甚至考虑过做这样的事情:这看起来是个好主意,实际上(经过一些清理):

class MyApp(object):
    def __init__(self):
        self.db1 = DB1()
        self.db2 = DB2()
    def __enter__(self):
        self.db1.__enter__()
        self.db2.__enter__()
        return self
    def __exit__(self, type, value, traceback):
        try:
            self.db1.__exit__(self, type, value, traceback)
        except:
            pass
        try:
            self.db2.__exit__(self, type, value, traceback)
        except:
            pass

编辑:修复了代码.

Answer 1

大多数上下文管理器都可以使用装饰器来编写@contextmanager。你编写一个只有一个yield的函数，yield之前是你的“进入”函数，yield之后是你的“退出”函数。由于生成器的实现方式，如果yield位于with语句中，则with语句不会在yield处退出。

例如。

from contextlib import contextmanager

class SomeContextManager:
    def __init__(self, name):
        self.name = name
    def __enter__(self):
        print("enter", self.name)
        return self
    def __exit__(self, ex_type, value, traceback):
        print("exit", self.name)

class SomeContextManagerWrapper:
    def __init__(self, *context_managers):
        self.context_managers = context_managers
    @property
    def names(self):
        return [cm.name for cm in self.context_managers]

@contextmanager
def context_manager_combiner():
    print("context_manager_combiner entering")
    with SomeContextManager("first") as a, SomeContextManager("second") as b:
        yield SomeContextManagerWrapper(a, b)
    print("context_manager_combiner exiting")

with context_manager_combiner() as wrapper:
    print("in with statement with:", wrapper.names)

输出：

context_manager_combiner entering
enter first
enter second
in with statement with: ['first', 'second']
exit second
exit first
context_manager_combiner exiting

Answer 2

我会使用第二个解决方案,但也处理数据库错误:

import sys

class MyApp(object):
    def __init__(self):
        self.db1 = DB1()
        self.db2 = DB2()
    def __enter__(self):
        self.db1.__enter__()
        try:
            self.db2.__enter__()
        except:
            self.db1.__exit__(None, None, None) # I am not sure with None
            raise
        return self
    def __exit__(self, type, value, traceback):
        try:
            self.db1.__exit__(self, type, value, traceback)
        finally:
            self.db2.__exit__(self, type, value, traceback)

第一个__exit__是__enter__因为with- 所以,不起作用.

编辑:也请查看@Ming的答案.在许多情况下,它更短.